1. Field of the Invention
The present invention relates to a white organic light emitting device (OLED) and a color display apparatus using the same, and more particularly, to a top emission type white OLED realizing high resolution and having a wide color gamut, and a color display apparatus using the same.
2. Description of the Related Art
An organic light emitting device (OLED) is a display device which forms images by emitting light when holes supplied from an anode and electrons supplied from a cathode are combined in an organic light emitting layer formed between an anode and a cathode. The OLED has excellent display characteristics such as a wide viewing angle, high response speed, a small thickness, low manufacturing costs, and high contrast ratio, and thus has recently become considered as a next-generation flat panel display device.
In general, in order to obtain improved light emitting characteristics, the OLED may have a multi-layered structure further including a hole injection layer and a hole transporting layer formed between the anode and the organic light emitting layer, and an electron injection layer and an electron transporting layer formed between the cathode and the organic light emitting layer. In addition, layers other than the above-described layers may be additionally formed.
The OLED may be designed to emit a desired colorful light by forming the organic light emitting layer by using an appropriate material. According to this principle, a color display apparatus may be realized using the OLED. For example, in a color display apparatus using the OLED, each pixel may be formed of a sub-pixel having an organic light emitting layer emitting a red color, a sub-pixel having an organic light emitting layer emitting a green color, and a sub-pixel having an organic light emitting layer emitting a blue color. Forming organic light emitting layers separately for each sub-pixel, however, causes manufacturing processes to become unnecessarily be more complicated, making the realization of a large surface and high resolution display device difficult.
Consequently, a white OLED has been suggested. A white OLED may be realized by forming a plurality of organic light emitting materials separately emitting red, green, and blue colors in an organic light emitting layer, or by forming pairs of two organic light emitting materials in a complementary color relationship. When using a white OLED, colors are attained by using color filters. In this case, the structure of the organic light emitting layer of all sub-pixels can be formed identically, and thus it is relatively easy to manufacture a large surface and high resolution display device.
Meanwhile, OLEDs may be classified into a bottom emission type OLED and a top emission type OLED, according to the direction in which light generated from an organic light emitting layer is emitted. In a bottom emission type OLED, light is extracted through a bottom surface on which a thin film transistor (TFT) driving the OLED is formed. On the other hand, in a top emission type OLED, a reflective electrode is formed in a lower portion of an organic light emitting layer and light is extracted through an upper portion of the TFT. In general, the top emission type OLED is more advantageous in terms of achieving high resolution since it can have a greater light emitting area (aperture ratio) than the bottom emission type OLED.
In the top emission type OLED, however, a resonance cavity is necessarily formed between the reflective electrode of the organic light emitting layer and a semi-transparent electrode in the upper portion of the organic light emitting layer. Resonance generated in such a resonance cavity narrows spectrums of light emitted to the exterior, and thus being advantageous in terms of extracting light of a predetermined wavelength only, but disadvantageous in terms of extracting white light.
Consequently, an OLED that outputs light with a predetermined wavelength in a single resonating mode has been suggested. In this case, however, the optical thickness of a resonance cavity must vary according to the light wavelengths. Accordingly, in order to manufacture a color display apparatus using such OLED, the optical thickness of the resonance cavity should be varied according to sub-pixels of each color, and this, in return, further complicates the manufacturing process.